In the past few years, I found that (1) the cerebellum of the premetamorphic frog tadpole is so immature that the external granular layer is altogether absent, (2) the tadpole cerebellum undergoes maturation at the time of metamorphosis, (3) these maturational changes can be induced precociously by either intraperitoneal injection of thyroxine or implants of thyroxine pellets in the 4th ventricle. We are currently studying the details of frog cerebellar maturation mainly by the Golgi silver impregnation procedure and electron microscopy.